1. Field of the Invention
The present disclosure relates to a flexible organic light emitting display device and, more particularly, to a flexible organic light emitting display device employing a wide band structure.
2. Background of the Invention
Recently, as interest in information displays has been on the rise and demand for the use of portable information media has been increased, lightweight flat panel displays (FPDs) substituting cathode ray tubes (CRTs) as existing display devices have been actively researched and commercialized.
In the FPD fields, a liquid crystal display (LCD) device, which is lighter and consumes less power, has been spotlighted; however, since an LCD device is a light receiving device, rather than a light emitting device, having shortcomings of brightness, contrast ratio, and a viewing angle, and the like, so a development of a new display device that may overcome such drawbacks has been actively made.
An LED display device, one of new display devices, is a self-luminous type device, which thus is excellent in a viewing angle and contrast ratio, is lighter and thinner because it does not need a backlight, and is advantageous in terms of power consumption, relative to an LCD device. In addition, an organic light emitting display device can be driven by a DC and at a low voltage, has a fast response speed, and is especially advantageous in terms of fabrication costs.
Unlike an LCD device or a plasma display panel (PDP), deposition and encapsulation are the whole of a fabrication process of an organic light emitting display device, so the fabrication process is very simple. Also, when the organic light emitting display device is driven according to an active matrix scheme in which each pixel has a thin film transistor (TFT) as a switching element, the same luminance can be obtained although a low current is applied, so, advantageously, the organic light emitting display device consumes low power, has a high pitch (or high definition or high resolution), and can be increased in size.
Hereinafter, a basic structure and operational characteristics of an organic light emitting display device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a light emission principle of a general organic light emitting display device.
As shown in FIG. 1, a general organic light emitting display device includes an organic light emitting diode (OLED). The OLED includes organic compound layers 19a, 19b, 19c, 19d, and 19e formed between an anode 18 as a pixel electrode and a cathode 8 as a common electrode.
Here, the organic compound layers 19a, 19b, 19c, 19d, and 19e include a hole injection layer 19a, a hole transport layer 19b, an emission layer 19c, an electron transport layer 19d, and an electron injection layer 19e. 
When a driving voltage is applied to the anode 18 and the cathode 8, holes which have passed through the hole transport layer 19b and electrons which have passed through the electron transport layer 19e move to the light emission layer 19c to form excitons, and as a result, the light emission layer 19c emits visible light.
In the organic light emitting display device, the pixels each having the OLED having the foregoing structure are arranged in a matrix form and selectively controlled by a data voltage and a scan voltage to display an image.
The organic light emitting display device is divided into a passive matrix type organic light emitting display device and an active matrix type organic light emitting display device using TFTs as switching elements. Among them, in the active matrix type organic light emitting display device, TFTs as active elements are selectively turned on to select pixels and emitting of pixels is maintained by a voltage maintained in a storage capacitor.
FIG. 2 is an equivalent circuit diagram of a pixel in a general organic light emitting display device. Namely, FIG. 2 illustrates an example of an equivalent circuit diagram of a pixel having a general 2T1C (including two transistors and one capacitor) in an active matrix type organic light emitting display device.
Referring to FIG. 2, a pixel of an active matrix type organic light emitting display device includes an OLED, a data line DL and a gate line GL crossing each other, a switching TFT SW, a driving TFT DR, and a storage capacitor Cst.
Here, the switching TFT SW is turned on in response to a scan pulse from the gate line GL to conduct a current path between a source electrode and a drain electrode thereof. During an ON-time period of the switching TFT SW, a data voltage from the data line DL is applied to a gate electrode of the driving TFT DR and the storage capacitor Cst by way of the source electrode and drain electrode of the switching TFT SW.
Here, the driving TFT DR controls a current flowing in the OLED according to the data voltage applied to the gate electrode thereof. The storage capacitor Cst stores a voltage between the data voltage and a low potential power source voltage VSS and uniformly maintains it during one frame period.
In order to prevent reflection of external light, the organic light emitting display device having such characteristics includes a polarizing plate attached to an upper surface thereof. The polarizing plate, having circular polarization, includes a linear polarizer and a quarter wave plate attached to an upper surface thereof.
FIG. 3A is a cross-sectional view illustrating a structure of a general organic light emitting display device.
Also, FIG. 3B is a cross-sectional view illustrating a structure of a polarizing plate of the general organic light emitting display device of FIG. 3A.
Referring to FIGS. 3A and 3B, the general organic light emitting display device includes a display panel 10 outputting an image, and a polarizing plate 30 is attached to an upper portion of the display panel 10 by using a first adhesive 31a. 
A protective film 35 as hard coating is attached to an upper surface of the polarizing plate 30.
Here, the display panel 10 is configured by using an organic light emitting element, and the polarizing plate 30 having circular polarization includes a linear polarizer 34 and a quarter wave plate 32 (or a λ/4 retardation plate).
Namely, in the case in which the display panel 10 is configured by using an organic light emitting element, subpixels are disposed in a matrix form in an active region implementing an image. In the display panel, a driving circuit unit 20 including thin film transistors (TFT) are formed on a TFT substrate 11 made of glass, and organic light emitting diode (OLEDs) including an emission layer R, G, and B, are formed thereon, to constitute subpixels.
In the display panel 10 configured as described above, a planarization film 15 and a thin film encapsulation layer 16 are formed in a upper portion thereof, and the polarizing plate 30 is attached to an upper portion of the thin film encapsulation layer 16 by using the first adhesive 31a. 
The polarizing plate 30 includes the linear polarizer 34, second and first supports 33b and 33a positioned in upper and lower portions of the linear polarizer 34, and the quarter wave plate 32 attached to the first support 33a through a second adhesive 31b. 
The organic light emitting display device is disadvantageous in that black reflectivity is increased because external light is reflected by a metal film used as an electrode. Thus, in order to prevent this, the organic light emitting display device employs the polarizing plate 30 including the quarter wave plate 32 as mentioned above. Namely, light made incident from the outside is linearly polarized by the linear polarizer 34 of the polarizing plate 30, and the linearly polarized light, passing through the quarter wave plate 32, is circularly polarized. The circularly polarized light is reflected by the metal film, and is linearly polarized, while passing through the quarter wave plate 32, and here, a phase difference between the linearly polarized light passing through the quarter wave plate 32 and the linearly polarized light when it was made incident is λ/2, so light does not come out.
However, a color shift occurs in a black screen due to the use of the quarter wave plate 32. This results in a significant difference in color sense according to directions and angles in and at which the display panel 10 is viewed, acting as a factor degrading quality of an actual product.